The present invention relates generally to reducing emissions from the exhaust of diesel engines and, more particularly, to the method, design and manufacture of monolithic cross flow traps for removing particulates from the exhaust of diesel and other internal combustion engines. This type of particulate trap is used in my co-pending application Ser. No. 09/516,480, filed Mar. 1, 2000, entitled xe2x80x9cApparatus and Method for Filtering Particulate In An Exhaust Trap now U.S. Pat. No. 6,233,926.xe2x80x9d
Diesel engine exhaust smoke is largely comprised of small soot particles or particulate that is a nuisance and which the Environmental Protection Agency has identified as a health hazard when inhaled into the lungs and this has resulted in federal regulations limiting the amount of particulate that an engine may emit into the atmosphere. Consequently, an intensive effort has been underway during the last twenty years to reduce the amount of particulate emitted into the atmosphere from diesel engine exhaust stacks. The preferred approach, and the one on which most effort has been expended, is to reduce the particulate generated by engines. Another approach has been to filter out or trap the particulate matter contained in the flow of exhaust gas prior to its release into the atmosphere. Because emission regulations have become ever more stringent, efforts have been made toward the development of practical and reliable particulate trap systems.
Particulate trap systems have generally consisted of two types: depth filters of metal wire coils or other material usually having a catalyst on the surfaces to promote ignition of the particulate and surface filters of ceramic or other heat resistant material. The catalyzed depth filters filter out or trap the particulate and promote its combustion. These are considered to be more expensive and, in addition, the catalyst can result in the generation of undesirable compounds in the exhaust emitted.
Most of the effort has been directed toward wall-flow trap systems that use porous ceramic that contain many passages, somewhat like a honeycomb as explained in Frost et al., U.S. Pat. No. 4,415,344 and shown in FIG. 1. The honeycomb section can be extruded; hence, relatively inexpensive to manufacture and provides a large passage surface area for a given size trap. By plugging the exit ends of alternate passages, and the entrance ends of the remaining passages, the exhaust gas is forced through the porous walls, of the in-flow passages into the out-flow passages as indicated by the arrows. The soot particles are removed from the dirty in-flow gas and collect in a layer which builds up on the inner walls of the passages; clean gas exits from the out-flow passages. While these traps remove 95-98% of the particulate from the exhaust gas stream, pressure drop across the trap increases due to the accumulation of the soot and ash. While the soot can be burned periodically by heating all or a portion of the exhaust gas, this entails considerable loss of energy and, more seriously, the heat of combustion of the soot leads to cracking and melting of the traps.
Complicated catalyst means have been used to lower the ignition temperature of the soot to protect the trap with mixed results but burnout does not prevent long term accumulation and pressure drop due to the build up of incombustible ash.
One system uses high pressure reverse flow of cool air to mechanically remove the soot and ash, and burn the soot outside the trap; however, the use of high pressure air requires a rather heavy structure trap system to provide adequate strength to prevent problems with the trap seals and valves used to control the reverse flow. In addition, the energy required to provide the high pressure air adversely affects engine efficiency.
As mentioned in my co-pending application Ser. No. 09/516,480, filed Mar. 1, 2000, entitled xe2x80x9cApparatus and Method for Filtering Particulate in an Exhaust Trap,xe2x80x9d there are cross flow traps, of the type shown in FIG. 2, that are made of porous ceramic tubes that pass through the trap module. End walls support the ceramic tubes and seal the spaces between them near their ends. As indicated by the arrows, a portion of the exhaust gas that enters the trap module at the left passes through the porous walls and the soot and ash is filtered out. The remaining or unfiltered exhaust gas is passed through the trap and exits at the right. The exhaust gas that passes through the porous walls passes through the small clearances between the tubes and exits through the space between the two end walls. The end walls of the trap module prevent unfiltered exhaust gas from entering the clearances.
Like the honeycomb wall-flow type of trap, the cross flow trap will remove 95 to 98% of the soot from the exhaust gas as the exhaust gas passes through the porous walls of the tubes. Also, in common with the better known honeycomb wall-flow trap, the filtering action leaves a layer of soot and ash on the inner surface of the tubular passages that will increase the pressure drop and engine back pressure; hence, adversely affecting engine performance. For this reason the layer of soot and ash must be periodically removed.
The inventions covered by my co-pending application use control techniques that cause all or a significant portion of the exhaust gas to periodically flow through and exit from the tubular passages at a velocity that is sufficient to dislodge and/or erode any significant accumulations of soot and ash. In all the embodiments, the dislodged particles are blown out of the passages to be burned and/or stored for periodic removal. By using these techniques, excellent filtration efficiency can be achieved and the soot and ash can be removed, burned and/or stored. Moreover, this can be accomplished without igniting the soot in the trap; hence, there are no adverse consequences, such as loss of trap life by cracking or melting.
The preferred embodiment of the cross flow multi-tubular trap module is to make the tubes and end walls as a monolith of a ceramic material such as porous cordierite. By maintaining a small but carefully controlled clearance between the individual tubes, the total internal surface area of the tubes per unit of trap module volume is about the same as the conventional honeycomb wall-flow trap. Thus, the total trap size and backpressure for a given engine are about the same and, by using the manufacturing techniques taught by this invention, can be made at equal or lower cost.
A tubular cross flow porous ceramic trap that has oval shaped passages is made by stacking formed sheets of cordierite has been developed by Asahi Glass Company and is the subject of Oda et al., U.S. Pat. No. 4,833,883. While this trap could be used as the particulate filter in the particulate trap system that is covered by my co-pending application, it is considered larger in size and more expensive to manufacture than the module configuration of this invention.
An objective of the invention is to provide monolithic cross flow trap designs that will provide high filtering efficiency and acceptable pressure loss in traps of minimal size and cost.
Another objective is to provide methods for manufacturing monolithic cross flow trap modules by extruding plasticized cordierite ceramic or other refractory materials in a single extrusion step followed by firing and minor finishing.
Yet another objective is to provide methods for economically manufacturing large size monolithic cross flow trap modules that are not easily manufactured by single step extrusion of the total trap module.
Still another objective is to provide methods for manufacturing monolithic cross flow trap modules of different sizes and configurations with minor changes in the manufacturing equipment.
In accordance with one aspect of the invention, there is provided a device for extruding plasticized cordierite ceramic forming material through a variable die section to form the trap. The die section can be arranged to form either a number of tubular passages having their outside surfaces in close proximity to each other or, by changing the configuration of the die set, to form passages in a simple honeycomb configuration. The multiple tubular passages have porous walls, that extend for nearly the full length of the trap module and comprise the filter section. The short honeycomb lengths at each end of the trap are made by thickening the tubular passage walls and provide the sealed end walls for separating the filtered exhaust from the unfiltered exhaust that enters the trap. Firing of the extruded trap produces a monolithic cross flow trap of porous ceramic that requires only minor finishing.
In accordance with another aspect of the invention, the manufacture of a monolithic cross flow trap begins with the conventional manufacture of tubes having walls of porous cordierite or similar material. The tubes are then woven into a mat using wire or mono-filament plastic as the binding means. Finally, the mat is rolled about a small mandrel while cords of pliable plasticized cordierite are fed between the turns near the ends of the tubes. The diameter of the wires or mono-filament provides a small closely controlled clearance between each tube while the plasticized cordierite cords squeezed around the outside diameters of the tubes form the two end walls of the trap module. The wound trap module is then fired and finish machined.
In yet another aspect of the invention, the porous cordierite tubes are formed with slightly enlarged outside diameters near their ends. When a number of the tubes are collected in a bundle, the enlarged diameters provide a closely controlled clearance between the outside diameters of the porous tubes throughout most of their lengths. The tubes, that have closed ends, are assembled into snug bundles and then end walls of pliable cordierite forming material are pressed over each end of the tubes. The monolithic trap module results from firing the module and machining of the end faces to remove the tube end closures and finish machine the trap faces.
These and other objects and advantages of the present invention will become apparent as the same becomes better understood from the following detailed description when taken in conjunction with the accompanying drawings.